1. Field of the Invention
The present invention relates to an image processing apparatus such as may be encountered in a digital printer, a digital facsimile, or the like, which handles an image as a digital signal, and also to an image processing method which is used in such an image processing apparatus. More particularly, the invention relates to an image processing method and apparatus for halftone processing of image data.
2. Related Background Art
Hitherto, as a binarizing method of reproducing a halftone in a digital printer, a digital facsimile, or the like, there has been known an error diffusion method whereby errors generated by a binarizing process are distributed to peripheral pixels. Such a method has been proposed in the paper by Floyd and Steinberg, "An Adaptive Algorithm for Spatial Grayscale", SID DIGEST. in 1975.
On the other hand, there has also been known a method called the method of least mean error. Such the method is considered to be equivalent to the error diffusion method.
In the case of performing the binarizing process by using the error diffusion method, since there is no periodicity in the error process, no moire occurs for a dotted image, and the resolution is better than that in the case of a dither method (an example of another binarizing method) or the like. However, there is a drawback that a unique fringe pattern is generated in highlight portions of the image. To eliminate the drawback of the error diffusion method as mentioned above, the assignee of the present invention has 145,593, 192,601, and 203,880,and 284,603.
On the other hand, in the case of performing the binarizing process for a white portion such as a background of characters or the like by the error diffusion method, there is a drawback that dots appear in the white portion.
To prevent the appearance of dots, the U.S. patent application Ser. No. 289,017.
In the conventional error diffusion method, there is a drawback that in the case where an A area of high concentration of an original and a B area of low concentration of the original are neighboring, as shown in FIG. 12, a (blanking) phenomenon, wherein no dot is printed in the area (hatched portion in FIG. 12) near the boundary between the A and B areas and consequently a reproduced image lacks the content it should have being blank instead, so that the image quality is remarkably deteriorated.